The present invention relates to a circular knitting machine with a device for repeating the selection of the needles at least at one thread feed.
As known, circular knitting machines are generally constituted by a needle cylinder which has, on its lateral surface, a plurality of grooves which extend along a direction which is substantially parallel to its axis. A needle and a selector arranged below said needle are accommodated in each of said grooves so as to be slidable in a direction which is parallel to the axis of the needle cylinder. Each selector can oscillate in a radial plane of the needle cylinder, by virtue of the action of selection means which are arranged laterally facing the needle cylinder, from an extraction position, in which a main heel of the selectors protrudes radially from the needle cylinder, to a sunk position, in which said main heel is contained within the radial dimensions of the needle cylinder. One or more actuation cams are arranged around the needle cylinder, proximate to the main heel of the selectors, and define paths which can be engaged by said main heel when the selector is in the extraction position. The paths defined by the actuation cams have rising portions, so as to move upward the selectors which are engaged with their main heel within said paths so that they act with their end on the overlying needle to move it into a position suitable for engaging the thread fed at a thread feed of the machine, or so as to cause the passage of the heel of the overlying needle, which protrudes radially from the needle cylinder and is engaged within paths defined by other actuation cams which are located at the level of the heel of the needles, from one path to the other so as to diversify the knitting of the needles according to requirements.
The selection means are generally constituted by a plurality of levers which are mutually stacked in a direction which is parallel to the axis of the needle cylinder and are individually movable so as to interfere or not interfere with selection heels located at mutually different levels along the longitudinal extension of the selector. Extraction means are arranged upstream of the selection levers according to the direction of the rotation imparted to the needle cylinder about its axis relatively to the actuation cams; said extraction means act on the selectors in a sunk position so as to move them into the extraction position in order to undergo the action of the selection levers. According to the actuation of given selection levers and to the positioning of the selection heels along the selectors, the passage of preset selectors from the extraction position to the sunk position is caused, whereas other selectors arranged below the needles which must be moved to take the thread at the immediately subsequent feed are kept in the extraction position so as to engage the actuation cams which cause the lifting of the selectors and therefore of the overlying needles.
During some types of knitting, the need is felt to be able to repeat a same needle selection at a plurality of successive feeds of the machine without necessarily using the selection means every time, in order to contain costs.
For this purpose, the Applicant has already provided a device, which is the subject of the Japanese patent application No. 58-067880, which comprises a memory element arranged proximate to the lower end of the selectors and can maintain the position of the selectors, previously set with the selection means upstream of a first feed, between the first feed and a second feed, so that the same needles which knitted at the first feed knit at the second feed without the need to perform a new selection between the two feeds. More particularly, said memory element comprises a fixed cam constituted by a raised portion on the resting plane of the lower end of the selectors. Said raised portion defines two diversified paths for the lower end of the selectors, and more particularly: a first path, arranged between the raised portion and the selector actuation cams, within which the lower end of the selectors which are in extraction position after the selection performed upstream of the first feed engages, and a second path, arranged between the raised portion and the needle cylinder, within which the lower end of the selectors which are in sunk position after said selection engages.
The raised portion is fixed to the resting plane of the lower end of the selectors. In order to obtain the repetition of the selection only when required, a cam is provided upstream of said raised portion. The cam is movable in a radial direction with respect to the needle cylinder, whereby to move the lower ends of all the selectors lying in the first path. In this way the cam returns the selectors from the sunk position to the extraction position, so that the previously performed selection is cancelled and the selectors can be selected by further selection means arranged between the first feed and the second feed.
Through the years, research in this field has been aimed mainly at obtaining high rotation rates of the needle cylinder in order to increase the productivity of these machines.
Due to this fact, the above described memory device has some disadvantages in modern circular knitting machines.
More particularly, the use of a movable cam followed by a fixed cam, in this kind of device, causes an excessive wear of the selectors at high speeds which compromises the correct operation of said selectors. Again due to the fact that the combination of a fixed cam and of a movable cam is used, in order to avoid excessive impacts between said cams and said selectors great precision is furthermore necessary both in the execution of these elements and in their registration during assembly.
Even a very precise execution and registration in any case do not avoid impacts at high speeds between the selectors and the movable cam, since the selectors in a sunk position tend to move into the extraction position due to centrifugal force.